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Why Dorian stalled out over the Bahamas

And why it’s on the move again

Florida Remains On Alert As Hurricane Dorian Nears Atlantic Coast Photo by NOAA via Getty Images

Hurricane Dorian’s projected course has shifted dramatically over the past few days, spreading a menacing uncertainty across the Caribbean, Florida, and the southeastern coast of the US. As the storm continues to lash the Bahamas with deadly force, changing weather patterns have pushed the storm around, leading to mounting frustrations as US residents struggle to prepare for the storm, and meteorologists try to pin down where it will go next.

After a stint as a powerful storm that smashed records across the Caribbean, Dorian’s winds have weakened to a Category 2 storm. But the hurricane has also grown in size. Over the next several days the most intense parts of the storm will stay over the Atlantic rather than making landfall in the US, but the National Hurricane Center warned that it would get “dangerously close” to Florida over the evening before it begins to crawl up the southeast coast.

The Verge spoke with meteorologists to understand what’s going on with Dorian’s path.

What makes Dorian stand out?

Dorian is giant, slow to move, and is lingering in an area packed with population centers. Its early track took it near Puerto Rico, which is still recovering from 2017’s Hurricane Maria, and for a while its projected path has loomed with uncertainty over the entire east coast of Florida, from Jacksonville to Miami.

“It’s unusual for a storm of this intensity to just stop, especially so close to highly populated areas, which has made it a real nail biter for everybody from the forecasters to the people along the southeast coast,” John A. Knox, professor of geography and undergraduate coordinator of the Atmospheric Sciences Program at the University of Georgia, tells The Verge.

Knox adds that “[It’s] not just a nail biter, but a real tragedy for the Bahamas.” CNN reports that it’s the strongest hurricane that’s ever made landfall in the Bahamas. The hurricane parked itself over the Bahamas as a Category 5 — the highest category on the Saffir-Simpson Hurricane Wind Scale — killing five people in what Prime Minister Hubert Minnis called, “a historic tragedy.”

So, why did Hurricane Dorian stall over the Bahamas?

In an email to The Verge, Jennifer McNatt, a meteorologist with the National Weather Service, explained that “the steering flow that typically moves systems like this was very weak.” Basically, the winds that typically steer a hurricane along fell apart at the same time that the storm landed over the Bahamas.

A “steering flow” is part of an overall wind pattern that can push or pull at smaller weather events, like hurricanes. “If those large-scale winds are weak, the hurricane won’t move much, which can make it more difficult to predict,” Allison Wing, assistant professor of meteorology at Florida State University, said to The Verge in an email.

Knox tells The Verge, “It’s kind of like being in a sailboat where the wind dies down, and then you’re stuck. You don’t go anywhere.”

Why has the predicted track of the storm changed so much?

As Dorian initially approached Florida, an atmospheric ridge — a peak of high pressure — was in place just to the north of Dorian in the Atlantic. In the Northern Hemisphere, air flows clockwise* around a ridge. So with a ridge to the north of Dorian, the winds would have likely pushed it west, directly into the Florida Coast.

Tropical storms like Dorian typically move slower than weather patterns further north in the Atlantic. With the storm already moving so gradually, the ridge had time to weaken and a “trough” of low pressure moved in toward the storm. The collapse of the ridge slowed the storm down even further, since it could no longer drive it west. Then the trough changed the hurricane’s direction.

“A trough is just the opposite,” Knox says. “It’s like a valley in pressure and the winds in the Northern Hemisphere blow counterclockwise around the trough. If you’re south of a trough, then you’re nudged in the east or northeast direction.” That trough is now drawing the hurricane away from the East Coast and out to sea.

Wing adds that small errors in assessing the strength or position of a large-scale pattern, like the ridge over the western Atlantic, or errors in the initial position of the hurricane can lead to big uncertainties in the forecast track.

Christina Koch/NASA

Does climate change have anything to do with it?

Over the past ten years, scientists have been better able to study what role climate change has played in affecting the likelihood or intensity of an extreme weather event. But it’s too soon to attribute Dorian’s strength or movement to climate change. That will require a lot more study.

While Dorian’s snail’s pace was unusual, it could be in line with expected changes in atmospheric circulation as global average temperatures rise. A study published in the journal Nature in June 2018 found that the forward motion of tropical cyclones slowed by 10 percent between 1949 and 2016. And that slowdown, according to the paper, is consistent with what scientists expect will take place as a result of human-caused climate change.

Hurricane Harvey was another devastating storm that lingered over Houston in 2017, dropping a trillion gallons of water over Harris County in a span of four days. “That’s one that’s in everybody’s minds as a large, major hurricane that didn’t go anywhere for a while. Dorian was an even stronger storm from a winds perspective and it also has parked. So is that a sign of things to come? No one can say for sure,” Knox says. But he points to the Nature paper which found the slower movement of storms like Harvey and Dorian to be consistent with conditions under a changing climate. In 2017, two independent studies found that Harvey’s rains were at least 15 percent heavier than they otherwise would have been if it wasn’t for climate change.

Has this storm been especially difficult to forecast?

Although Dorian’s path has shifted, the National Weather Service has promptly updated its projections as new factors pushed and pulled at the storm. “I wouldn’t necessarily say that Dorian has been especially difficult to forecast,” McNatt with the National Weather Service wrote to The Verge. “As expected, with updated model data and real-time observational data, the forecasts have evolved since advisories began.”

National Weather Service forecasts build on models and real-time observations collected from offices throughout the country. Twice a day, McNatt said, 92 weather stations across North America and the Pacific, plus 10 stations in the Caribbean release weather balloons into the atmosphere. Those balloons collect data on temperature, wind speed, humidity, and pressure for the NWS.

A satellite image of Dorian.
NASA/NOAA/UWM-SSEC-CIMSS/William Straka III

That data is fed into computer models that figure out where the storm might end up based on the movement of air and moisture in the atmosphere. The range of possible outcomes for any particular storm is, not surprisingly, wider for predictions further in the future, but narrows as a storm gets closer.

And there are a lot of factors that can make predicting the path of any storm challenging. For one, they form over the ocean where scientists can’t rely on surface weather observations. Occasionally they’ll get data from passing ships, and so-called “hurricane hunter” aircraft can fly into the storm to make observations, with obvious risks for both sailors and pilots. This is where satellite data has changed the game, allowing scientists to better see into storms out at sea without endangering people’s lives.

Wing at Florida State University also points to how ambiguity in estimates of Dorian’s strength might also affect how forecasters figure out where it’s headed next. “Uncertainty in the intensity of the hurricane can also impact the track forecast, as intensity affects how a hurricane interacts with the large-scale wind pattern,” Wing wrote to The Verge in an email. “There are still gaps in our understanding of what controls hurricane intensity. In particular, forecasting rapid intensification remains very challenging.”

Still, Knox considers the national forecasts for Dorian a success. Accurate forecasts not only give people fair warning, they also prevent unnecessary chaos caused by evacuating areas that won’t actually get hit. “Despite what people might think, the official forecasts have been really good,” he says.

*Correction 9/4: A previous version of this article stated that in the Northern Hemisphere, air flows counterclockwise around a ridge. It flows clockwise. We regret the error.